This invention relates to continuous casting of thin steel strip in a twin roll caster. More specifically, this invention relates to the operation of and reduction of wear in side dams.
In a twin roll caster, molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are internally cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a thin cast strip product, delivered downwardly from the nip between the casting rolls. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal may be poured from a ladle through a metal delivery system comprised of a tundish and a core nozzle located above the nip, to form a casting pool of molten metal supported on the casting surfaces of the rolls above the nip and extending along the length of the nip. This casting pool is usually confined between refractory side plates or dams held in sliding engagement with the end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.
When casting steel strip in a twin roll caster, the thin cast strip leaves the nip at very high temperatures, of the order of 1400° C. If exposed to normal atmosphere, it will suffer very rapid scaling due to oxidation at such high temperatures. A sealed enclosure is therefore provided beneath the casting rolls to receive the hot cast strip, and through which the strip passes away from the strip caster, which contains an atmosphere that inhibits oxidation of the strip. The oxidation inhibiting atmosphere may be created by injecting a non-oxidizing gas, for example, an inert gas such as argon or nitrogen, or combustion exhaust reducing gases. Alternatively, the enclosure may be sealed against ingress of an ambient oxygen-containing atmosphere during operation of the strip caster, and the oxygen content of the atmosphere within the enclosure reduced, during an initial phase of casting, by allowing oxidation of the strip to extract oxygen from the sealed enclosure as disclosed in U.S. Pat. Nos. 5,762,126 and 5,960,855.
The length of the casting campaign has been generally determined in the past by the wear cycle on the core nozzle, tundish and side dams. Multi-ladle sequences can be continued so long as the source of hot metal supplies ladles of molten steel, which can be transferred into and out of the operating position by use of a turret. Therefore, the focus of attention to lengthen casting campaigns has been extending the life cycle of the core nozzle, tundish and side dams. When a nozzle, tundish or side dam wears to the point that it has to be replaced, the casting campaign has to be stopped, and the worn out component replaced. This would generally require removing unworn components as well since otherwise the length of the next campaign would be limited by the remaining useful life of the worn but not replaced refractory components, with attendant waste of useful life of refractories and increased cost of casting steel. Further, all of the refractory components would have to be preheated before the next casting campaign can start. Graphitized alumina, boron nitride and boron nitride-zirconia composites are examples of suitable refractory materials for metal delivery components. Since the core nozzle, tundish and side dams all have to be preheated to very high temperatures approaching that of the molten steel, there can be considerable waste of casting time between campaigns. See U.S. Pat. Nos. 5,184,668 and 5,277,243.
The present invention limits down time for changes of worn refractory components, decreases waste of useful life of refractory components, reduces energy needs in casting, and increases casting capacity of the caster. Useful life of refractories can be increased, and reheating of unreplaced refractory components can be avoided or minimized. The core nozzle must be put in place before the tundish, and conversely the tundish must be removed before core nozzle can be replaced, and both of these refractory components wear independently of each other. Similarly, the side dams wear independently of the core nozzles and tundish, and independently of each other, because the side dams must initially be urged against the ends of the casting rolls under applied forces, and “bedded in” by wear so as to ensure adequate sealing against outflow of molten steel from the casting pool. The forces applied to the side dams may be reduced after an initial bedding-in period, but will always be such that there is significant wear of the side dams throughout the casting operation. For this reason, the core nozzle and tundish in the metal delivery system can have a longer life than the side dams, and can normally continue to be operated through several more ladles of molten steel supplied in a campaign. Thus the duration of a casting campaign is usually determined by the rate of wear of the side dams however the tundish and core nozzle, which still have useful life, are often changed when the side dams are changed to increase casting capacity of the caster. No matter which refractory component wears out first, a casting run will need to be terminated to replace the worn out component. Since the cost of thin cast strip production is directly related to the length of the casting time, unworn components in the metal delivery system are generally replaced before the end of their useful life as a precaution to avoid further disruption of the next casting campaign, with attendant waste of useful life of refractory components.
By the present invention, it is possible to extend casting campaign lengths by minimizing side dam wear and thus, reducing waste of refractory components, operating costs and increasing casting time.
A method of continuous casting thin strip is disclosed comprising the steps of:                a. assembling a pair of casting rolls laterally positioned to form casting pool of molten supporting on casting surfaces of the casting rolls confined by side dams adjacent opposite ends surfaces of the casting rolls metal, and a nip between the casting rolls through which cast strip can discharge downwardly,        b. at the start of a casting campaign, pressing the side dams against the end surfaces of the casting rolls such that the side dams exert a pressure against the end surfaces of the casting rolls of less than 3.0 kg/cm2 but more than 1.25 kg/cm2, and        c. after the target casting pool height is reached, reducing the pressure exerted by the side dams against the end surfaces of the casting rolls to below 1.25 kg/cm2 to reduce wear of the side dams against the end surfaces of the casting rolls, while resisting ferrostatic pressure from the casting pool.        
At the start of a casting campaign, the pushing force may be greater than 1.5 kg/cm2 or greater than 1.9 kg/cm2. After the target casting pool height is reached, the pressure exerted by the side dams against the end surfaces of the casting rolls may be below 0.5 kg/cm2 or below 0.25 kg/cm2.
The wear rate of the side dams during casting after the target pool height is reached may range from 0.0001 mm/sec to 0.005 mm/sec, or may range from 0.0008 mm/sec to 0.0032 mm/sec.